A variety of electronic circuits are used to drive diode loads and, more particularly, to control electrical current through strings of series connected light-emitting diodes (LEDs), which, in some embodiments, form an LED display. It is known that individual LEDs have a variation in forward voltage drop from unit to unit. Therefore, the strings of series connected LEDs can have a variation in forward voltage drop.
Strings of series connected LEDs can be coupled to a common boost switching regulator at one end of the strings, the boost switching regulator configured to provide a high enough voltage to supply each of the strings of LEDs. The other end of each of the strings of series connected LEDs can be coupled to a respective current sink, configured to sink a relatively constant current through each of the strings of series connected LEDs.
It will be appreciated that the voltage generated by the common boost switching regulator must be a high enough voltage to supply the one series connected string of LEDs having the greatest total voltage drop, plus an overhead voltage needed by the respective current sink. In other words, if four series connected strings of LEDs have voltage drops of 30V, 30V, 30V, and 31 volts, and each respective current sink requires at least one volt in order to operate, then the common boost switching regulator must supply at least 32 volts.
While it is possible to provide a fixed voltage boost switching regulator that can supply enough voltage for all possible series strings of LEDs, such a boost switching regulator would generate unnecessarily high power dissipation when driving strings of series connected LEDs having less voltage drop. Therefore, in some LED driver circuits, the voltage drops through each of the strings of series connected LEDs are sensed and the common boost switching regulator is controlled to generate an output voltage only high enough to drive the series connected LED string having the highest voltage drop.
While the above-described electronic technique can result in a reduction of power dissipation, the above-described electronic technique can also suffer a high power dissipation if one of the series connected strings of LEDs becomes open circuit, i.e., fails. In this situation, a high voltage drop would be sensed and the common boost switching regulator would be controlled to increased its output voltage as high as it is able, resulting in a higher power dissipation associated with the remaining strings of series connected LEDs.